CORBA, DCOM, and Globe all provide distributed object models but have different design goals and implementations. CORBA aims for interoperability and provides many standardized services, while DCOM focuses on functionality within Windows environments. Globe emphasizes scalability through replication-based fault tolerance and location transparency using a global naming service. All support synchronous communication but Globe does not provide asynchronous messaging or callbacks like CORBA and DCOM. Security approaches also differ, with Globe requiring more work to support standardized mechanisms.

1. Distributed Object-Based
Systems
Chapter 9

2. Overview of CORBA
The global architecture of CORBA.

3. Object Model
The general organization of a CORBA system.

4. Corba Services
Overview of CORBA services.
Service Description
Collection Facilities for grouping objects into lists, queue, sets, etc.
Query Facilities for querying collections of objects in a declarative manner
Concurrency Facilities to allow concurrent access to shared objects
Transaction Flat and nested transactions on method calls over multiple objects
Event Facilities for asynchronous communication through events
Notification Advanced facilities for event-based asynchronous communication
Externalization Facilities for marshaling and unmarshaling of objects
Life cycle Facilities for creation, deletion, copying, and moving of objects
Licensing Facilities for attaching a license to an object
Naming Facilities for systemwide name of objects
Property Facilities for associating (attribute, value) pairs with objects
Trading Facilities to publish and find the services on object has to offer
Persistence Facilities for persistently storing objects
Relationship Facilities for expressing relationships between objects
Security Mechanisms for secure channels, authorization, and auditing
Time Provides the current time within specified error margins

5. Object Invocation Models
Invocation models supported in CORBA.
Request type Failure semantics Description
Synchronous At-most-once Caller blocks until a response is
returned or an exception is
raised
One-way Best effort delivery Caller continues immediately
without waiting for any response
from the server
Deferred
synchronous
At-most-once Caller continues immediately
and can later block until
response is delivered

6. Event and Notification Services (1)
The logical organization of suppliers and consumers
of events, following the push-style model.

7. Event and Notification Services (2)
The pull-style model for event delivery in CORBA.

8. Messaging (1)
CORBA's callback model for asynchronous method invocation.

9. Messaging (2)
CORBA'S polling model for
asynchronous method invocation.

10. Interoperability
GIOP message types.
Message type Originator Description
Request Client Contains an invocation request
Reply Server Contains the response to an invocation
LocateRequest Client Contains a request on the exact location of an object
LocateReply Server Contains location information on an object
CancelRequest Client Indicates client no longer expects a reply
CloseConnection Both Indication that connection will be closed
MessageError Both Contains information on an error
Fragment Both Part (fragment) of a larger message

11. Clients
Logical placement of interceptors in CORBA.

12. Portable Object Adaptor (1)
Mapping of CORBA object identifiers to servants.
a) The POA supports multiple servants.
b) The POA supports a single servant.

13. Portable Object Adaptor (2)
Changing a C++ object into a CORBA object.
My_servant *my_object; // Declare a reference to a C++ object
CORBA::Objectid_var oid; // Declare a CORBA identifier
my_object = new MyServant; // Create a new C++ object
oid = poa ->activate_object (my_object);
// Register C++ object as CORBA OBJECT

14. Agents
CORBA's overall model of agents, agent systems, and regions.

15. Object References (1)
The organization of an IOR with specific information for IIOP.

16. Object References (2)
Indirect binding in CORBA.

17. Caching and Replication
The (simplified) organization of a DCS.

18. Object Groups
A possible organization of an IOGR for an object
group having a primary and backups.

19. An Example Architecture
An example architecture of a fault-tolerant CORBA system.

20. Security (1)
The general organization for secure object invocation in CORBA.

21. Security (2)
The role of security interceptors in CORBA.

22. Overview of DCOM
The general organization of ActiveX, OLE, and COM.

23. Object Model
The difference between language-defined and binary interfaces.

24. Tape Library and Registry
The overall architecture of DCOM.

25. DCOM Services
Overview of DCOM services in comparison to CORBA services.
CORBA Service DCOM/COM+ Service Windows 2000 Service
Collection ActiveX Data Objects -
Query None -
Concurrency Thread concurrency -
Transaction COM+ Automatic Transactions Distributed Transaction Coordinator
Event COM+ Events -
Notification COM+ Events -
Externalization Marshaling utilities -
Life cycle Class factories, JIT activation -
Licensing Special class factories -
Naming Monikers Active Directory
Property None Active Directory
Trading None Active Directory
Persistence Structured storage Database access
Relationship None Database access
Security Authorization SSL, Kerberos
Time None None

26. Events
Event processing in DCOM.

27. Clients
Passing an object reference in DCOM with custom marshaling.

28. Monikers (1)
Binding to a DCOM object by means of file moniker.
Step Performer Description
1 Client Calls BindMoniker at moniker
2 Moniker
Looks up associated CLSID and instructs SCM
to create object
3 SCM Loads class object
4 Class object
Creates object and returns interface pointer to
moniker
5 Moniker Instructs object to load previously stored state
6 Object Loads its state from file
7 Moniker Returns interface pointer of object to client

29. Monikers (2)
DCOM-defined moniker types.
Moniker type Description
File moniker Reference to an object constructed from a file
URL moniker Reference to an object constructed from a URL
Class moniker Reference to a class object
Composite moniker Reference to a composition of monikers
Item moniker Reference to a moniker in a composition
Pointer moniker Reference to an object in a remote process

30. Active Directory
The general organization of Active Directory.

31. Fault Tolerance
Transaction attribute values for DCOM objects.
Attribute value Description
REQUIRES_NEW A new transaction is always started at each invocation
REQUIRED A new transaction is started if not already done so
SUPPORTED Join a transaction only if caller is already part of one
NOT_SUPPORTED Never join a transaction
DISABLED Never join a transaction, even if told to do so

32. Declarative Security (1)
Authentication levels in DCOM.
Authentication level Description
NONE No authentication is required
CONNECT Authenticate client when first connected to server
CALL Authenticate client at each invocation
PACKET Authenticate all data packets
PACKET_INTEGRITY Authenticate data packets and do integrity check
PACKET_PRIVACY Authenticate, integrity-check, and encrypt data packets

33. Declarative Security (2)
Impersonation levels in DCOM.
Impersonation level Description
ANONYMOUS The client is completely anonymous to the server
IDENTIFY
The server knows the client and can do access control
checks
IMPERSONATE The server can invoke local objects on behalf of the client
DELEGATE The server can invoke remote objects on behalf of the client

34. Programmatic Security
a) Default authentication services supported in DCOM.
b) Default authorization services supported in DCOM.
Service Description
NONE No authentication
DCE_PRIVATE DCE authentication based on shared keys
DCE_PUBLIC DEC authentication based on public keys
WINNT Windows NT security
GSS_KERBEROS Kerberos authentication
(a)
Service Description
NONE No authorization
NAME Authorization based on the client's identity
DCE Authorization using DEC Privilege Attribute Certificates (PACs)
(b)

35. Globe Object Model (1)
The organization of a Globe distributed shared object.

36. Globe Object Model (2)
The general organization
of a local object for
distributed shared
objects in Globe.

37. Globe Object Model (3)
Interfaces implemented by the semantics subobject of
a GlobeDoc object.
Document Interface
Method Description
AddElement Add an element to the current set of elements
DeleteElement Remove an element from the Web document
AllElements Return a list of the elements currently in the document
SetRoot Set the root element
GetRoot Return a reference to the root element
Content Interface
Method Description
GetCotent Return the content of an element as an array of bytes
PutContent Replace the content of an element with a given array of bytes
PutAllContent Replace the content of an entire document

38. Globe Object Model (4)
Interfaces implemented by the semantics
subobject of a GlobeDoc Object.
Property Interface
Method Description
GetProperties Return the list of (attribute, value)-pairs of an element
SetProperties Provide a list of (attribute, value)-pairs for an element
Lock Interface
Method Description
CheckOutElements Check out a series of elements that require modification
CheckInElements Check in a series of modified elements
GetCheckedElements Get a list of elements that are currently checked out

39. Process-to-Object Binding
Binding a process to an object in Globe.

40. Globe Services
Overview of possible Globe implementations of typical
distributes-systems services.
Service Possible Implementation in Globe Available
Collection Separate object that holds references to other objects No
Concurrency Each object implements its own concurrency control strategy No
Transaction Separate object representing a transaction manager No
Event/Notification Separate object per group of events (as in DCOM) No
Externalization Each object implements its own marshaling routines Yes
Life cycle Separate class objects combined with per-object implementations Yes
Licensing Implemented by each object separately No
Naming Separate service, implemented by a collection of naming objects Yes
Property/Trading Separate service, implemented by a collection of directory objects No
Persistence Implemented on a per-object basis Yes
Security Implemented per object, combined with (local) security services Yes
Replication Implemented on a per-object basis Yes
Fault tolerance Implemented per object combined with fault-tolerant services Yes

41. Communication
Invoking an object in Globe that uses active replication.

42. Globe Server
Operations on a Globe object server.
Method Description
Bind Lets the server bind to a given object, unless it is already bound
AddBinding Lets the server bind to an object, even if it is already bound
CreateLR Lets the server create a local object for a new distributed object
RemoveLR Lets the server remove a local object of a given object
UnbindDSO Lets the server remove all local objects of a given object
ListAll Returns a list of all local objects
ListDSO Returns a list of all local objects for a given objects
StatLR Get the status of a specific local object

43. Object References and Contact Addresses (1)
The representation of a protocol layer in a stacked contact address.
Field Description
Protocol identifier A constant representing a (known) protocol
Protocol address A protocol-specific address
Implementation handle Reference to a file in a class repository

44. Object References and Contact Addresses (2)
The representation of an instance contact address.
Field Description
Implementation handle Reference to a file in a class repository
Initialization string String that is used to initialize an implementation

45. Globe Naming Service
Iterative DNS-based name resolution in Globe.

46. Replication (1)
The interface of the replication subobject as
made available to the control subobject.
Method Description
Start Indicate that a new method invocation has been locally requested
Send Pass the marshaled invocation request to the replication subobject
Invoked Indicate that the invocation on the semantics object has completed

47. Replication (2)
The behavior of the control
subobject as a finite state machine.

48. Examples of Replication in Globe (1)
State transitions and actions for active replication.
Read method
State Action to take Method call Next state
START None Start INVOKE
INVOKE Invoke local method Invoked RETURN
RETURN Return results to caller None START
Modify method
State Action to take Method call Next state
START None Start SEND
SEND Pass marshaled invocations Send INVOKE
INVOKE invoke local method Invoked RETURN
RETURN Return results to caller None START

49. Examples of Replication in Globe (2)
State transitions and actions with primary-backup replication.
Read method
State Action to take Method call Next state
START None Start INVOKE
INVOKE Invoke local method Invoked RETURN
RETURN Return results to caller None START
Modify method at backup replica
State Action to take Method call Next state
START None Start SEND
SEND Pass marshaled invocation Send RETURN
RETURN Return results to caller None START
Modify method at primary replica
State Action to take Method call Next state
START none Start INVOKE
INVOKE invoke local method Invoked RETURN
RETURN Return results to caller None START

50. Security (1)
The position of a security subobject in a Globe local object.

51. Security (2)
Using Kerberos to establish
secure distributed shared
objects.

52. Summary (1)
Comparison of CORBA, DCOM, and Globe.
Issue CORBA DCOM Globe
Design goals Interoperability Functionality Scalability
Object model Remote objects Remote objects Distributed objects
Services Many of its own From environment Few
Interfaces IDL based Binary Binary
Sync. communication Yes Yes Yes
Async. communication Yes Yes No
Callbacks Yes Yes No
Events Yes Yes No
Messaging Yes Yes No
Object server Flexible (POA) Hard-coded Object dependent
Directory service Yes Yes No
Trading service yes No No
Continued …

53. Summary (2)
Comparison of CORBA, DCOM, and Globe.
Issue CORBA DCOM Globe
Naming service Yes Yes Yes
Location service No No Yes
Object reference Object's location Interface pointer True identifier
Synchronization Transactions Transactions Only intra-object
Replication support Separate server None Separate subobject
Transactions Yes Yes No
Fault tolerance By replication By transactions By replication
Recovery support Yes By transactions No
Security Various mechanisms Various mechanisms More work needed